## Key Idea: If a force acts on an object in the same direction as the direction of its motion, the object’s speed will continue to increase while the force is acting.-and-If a force acts on an object in the direction opposite to the direction of the object’s motion, the object’s speed will continue to decrease while the force is acting.

These items have been aligned to more than one key idea. To view the sub-ideas click on a key idea below.

• If a force acts on an object in the same direction as the direction of its motion, the object’s speed will continue to increase while the force is acting.

Students are expected to know that:

1. If a force is acting on an object to push or pull it forward, the object will continue to move faster and faster in the forward direction the entire time the force is acting.
2. If an object’s speed is increasing, then a force must be acting on the object the entire time its speed is increasing.

Boundaries:

1. Students are expected to analyze situations involving no more than two forces acting on an object at the same time that act along the object’s line of motion. Students are not expected to analyze situations in which the force is acting at an angle other than along the object’s line of motion, which would change the direction of the object’s motion.
2. Test items will involve situations in which forces are constant, not situations in which the forces are increasing or decreasing.
• If a force acts on an object in the direction opposite to the direction of the object’s motion, the object’s speed will continue to decrease while the force is acting.

Students are expected to know that:

1. If a force, either constant or changing, acts on an object in the direction opposite to the direction of the object’s motion, the object’s speed will decrease and will continue to decrease for as long as that force is greater than any force moving the object forward.
2. If an object’s speed is decreasing, a force must be acting on the object in the direction opposite to the direction of the object’s motion, and that opposing force must be greater than any force moving the object forward.
3. If a force acts on an object in the direction opposite to the direction of the object’s motion for a long enough time, the object’s speed will decrease to zero. If the same force continues to act, the object will move in the direction opposite to its previous motion.

Boundaries:

1. Students are expected to analyze situations involving no more than two forces acting on an object at the same time, and each force must act along the object’s line of motion or, if the object is not moving, the forces are acting along the same line.
2. Test items will involve situations in which forces are constant, not situations in which the forces are increasing or decreasing.
Percent of students answering correctly (click on the item ID number to view the item and additional data)
Item ID
Number
Knowledge Being Assessed Grades
6–8
9–12
Select This Item for My Item Bank

42%

52%

31%

34%

Frequency of selecting a misconception

Misconception
ID Number

Student Misconception

6–8
9–12

FMM125

When a force acts on a moving object to slow the object down, the object will slow down for a while and then move at a lower constant speed (AAAS Project 2061, n.d.).

N/A

N/A

FMM124

When a force acts on a moving object in the direction opposite the object’s direction of motion, the object will move at a constant speed for a while and then slow down (AAAS Project 2061, n.d.).

N/A

N/A

Frequency of selecting a misconception was calculated by dividing the total number of times a misconception was chosen by the number of times it could have been chosen, averaged over the number of students answering the questions within this particular idea.